1. Field of the Invention
The present invention relates to the circuit configuration and layout of a pixel area of an active matrix display device in which thin-film transistors are used and source lines are located over gate lines. In particular, the invention relates to the structure of an auxiliary capacitor.
2. Description of the Related Art
In recent years, because of increased demand for the active matrix liquid crystal display device, the techniques of forming thin-film transistors (TFTs) on an inexpensive glass substrate have been developed at high speed. In the active matrix liquid crystal display device, thin-film transistors are provided for tens to hundreds of millions of pixels arranged in matrix form, respectively, and charge to enter or exit from each pixel electrode is controlled by the switching function of the associated thin-film transistor.
A liquid crystal is interposed between each pixel electrode and the opposed electrode to constitute a capacitor. Therefore, by controlling charge to enter or exit from the capacitor with the thin-film transistor, the electro-optical characteristic of the liquid crystal is varied and the light passing through the liquid crystal panel is thereby controlled. Image display is performed in this manner.
However, the above-configured capacitor has a problem that the voltage held by itself gradually decreases as time elapses due to current leakage and a resulting variation in electro-optical characteristic deteriorates the contrast of image display. To solve this problem, another capacitor called "auxiliary capacitor" is commonly used to supply the capacitor including the liquid crystal with charge corresponding to charge that has been lost by leakage etc.
FIG. 1 is a circuit diagram of a conventional active matrix liquid crystal display device. The active matrix display circuit is generally divided into three parts: a gate driver circuit 2 for driving gate lines (scanning lines) 4, a data driver circuit 1 for driving source lines (data lines or signal lines) 5, and an active matrix circuit 3 in which pixels are provided. The data driver circuit 1 and the gate driver circuit 2 are generically called "peripheral circuits."
In the active matrix circuit 3, a large number of gate lines 4 and source lines 5 are provided so as to cross each other and pixel electrodes 7 are provided at their respective intersections. Switching elements (thin-film transistors) 6 are provided to control charges to enter or exit from the respective pixel electrodes. Either the top-gate thin-film transistor (the gate electrode is located over the active layer) or the bottom-gate thin-film transistor (the active layer is located over the gate electrode) is used depending on the desired circuit structure, manufacturing process, characteristics, and other factors. As described above, auxiliary capacitors 8 are provided parallel with the respective pixel capacitors to prevent voltage variations in the pixels due to leak current (see FIG. 1).
The conductivity of the thin-film transistor is varied when it is illuminated with light. To prevent this phenomenon, it is necessary to cover the thin-film transistor with a coating (black matrix or black mask) having light interrupting ability. The light-interruptive film needs to be formed also to prevent color or brightness mixture between adjacent pixels and a display failure due to electric field disorder at pixel boundaries.
For the above reasons, the light-interruptive film (hereinafter referred to as "light shield film") assumes a matrix form and is therefore called a black matrix (BM). Although at first the BM was provided on the substrate (opposed substrate) that confronts the substrate on which the active matrix circuit is provided because of advantages in a manufacturing process, it is now proposed that the BM be provided on the substrate on which the active matrix circuit is provided because of the need for increasing the area of each pixel (i.e., aperture ratio).
Although various proposals have been made of the structure of the auxiliary capacitor, it is difficult to obtain large capacitance while maintaining the pixel opening area (light transmitting area).